Pump

ABSTRACT

A gear pump includes a pump body  22  which defines a cavity hole  72  which holds a separate cavity body. The cavity body defines channels and a pump cavity holding a pair of gears. An inlet and outlet are connected to the channels. The separate cavity body may be manufactured separately and only this component needs to be manufactured to very accurate tolerances.

The invention relates to a gear pump and method of manufacture.

An important application for pumps is in the medical field. Where the fluid being processed is a bodily fluid such as blood or peritoneal fluid, pumps need to avoid cross-contamination from one patient to another. For this reason, medical equipment frequently uses pumps in which fluid flows through a flexible tube and is urged forwards by rollers outside the tube. The flexible tube can then be replaced for each patient.

However, such pumps are not suitable in all circumstances.

A gear pump includes two interlocked gears each mounted on a separate shaft in a cavity that is sized to be slightly larger than the gears. One of the gear shafts is driven to rotate the gears and liquid is entrained on the outside of the gears and driven from an inlet to an outlet. However, conventional gear pumps are relatively difficult to manufacture since the tolerances for the gap between the gears and the cavity are very tight. If the gears touch the cavity, then the motion of the gear is impeded, and if the gap is too large little fluid pressure is achieved. The shafts therefore need to be rigid.

Such manufacturing difficulties are particularly difficult when manufacturing small pumps since the tight tolerances required are very difficult to achieve.

There is accordingly a need for pumps which can be manufactured at relatively low cost.

According to the invention there is provided a pump system comprising a pump body having a pair of gears located in a pump cavity and defining channels, an inlet and an outlet connected to the channels of the pump cavity; and a motor connected to a longitudinal shaft for driving the gears; wherein one of the gears is a free gear located in the pump cavity by the walls of the cavity, and the pump body defines a cavity hole and the pump cavity and channels are defined in a cavity block mounted in the cavity, the cavity block being free to move laterally in the cavity hole.

By providing a pump system as described manufacture is eased. In particular, by providing the pump cavity in a separate cavity block that fits within the pump body, the cavity block can be manufactured to fine tolerances and if necessary with expensive materials without needing to increase the expense of manufacturing the pump body and the cover.

In a preferred embodiment, the cavity block is free to move laterally within the pump body. This avoids the need for accurate alignment and hence avoids the need to machine the pump body to fine tolerances. Also, it ensures that the components are not forced into an incorrect position by small inconsistencies in the manufacture of the pump body.

Further, the gears may be located within the cavity block, including one idle gear without a shaft and one drive gear with a shaft. Preferably, the drive gear is located within the cavity block by the gear within the cavity, the accurate alignment of the drive gear and drive shaft with the motor being possible by virtue of the freedom of the cavity block to move in the pump body.

The cavity block may have channels extending through the complete block and the cavity shaped for the two gears may likewise extend through the complete block, both having a constant cross section. In this way, manufacture of the cavity is eased since it is generally easier to manufacture through holes of constant shape.

Sealing of the cavity may be provided by a top pad over the cavity block and a bottom pad under the cavity block. In a preferred embodiment, the top and bottom pad have a relatively low elasticity (i.e. are made from rigid plastics).

Holes may be provided in the top pad to align with the channels to pass fluid and hose connectors may be provided extending through the cover and the holes in the top pad into the channels.

A shallow groove may be provided in the cavity block between each channel and the cavity. The groove may be shallow enough to pass fluid without interfering significantly with the location of the gears in the cavity.

The pump system may include a separate drive unit, comprising a housing, at least one motor and at least one drive disk connected to a respective motor to be rotated by the motor; and at least one removable pump unit, having the body, the cavity block, the gears in the cavity in the cavity block, and the cover over the cavity block.

The pump system may further include a driven disk connected to the pair of gears arranged to mesh with a drive disk connected to the motor to pick up power from the drive unit to drive the gears to pump fluid from the inlet to the outlet.

By providing a pump system as described a reliable system is achieved that minimises the risk of contamination by providing removable units.

For a better understanding of the invention, embodiments will now be described, purely by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows an exploded view of a first embodiment of the invention;

FIG. 2 shows a detail of the cavity block of FIG. 1;

FIG. 3 shows FIG. 2 with the gears mounted;

FIG. 4 shows a side view of the removable part of the arrangement of FIG. 1;

FIG. 5 shows an exploded view of a second embodiment of the invention; and

FIG. 6 shows an exploded view of a third embodiment of the invention.

Like components in different embodiments are given the same reference numbers and the corresponding description may be omitted.

Referring to FIG. 1, a pump system includes a drive unit 2 having housing 4. An electric drive motor 6 is provided, which drives a drive disk 8. A top piece 12 sits above the motor 6.

The pump system also includes a removable and replaceable pump unit 20. A pump body 22 is provided, which contains a separate cavity block 70 defining pump cavity 24 (see FIG. 2). The cavity block 70 may also be referred to as a volute.

The pumping action is provided by a pair of gears 30, 32 in the pump cavity 24 (see FIG. 3). One of the gears is a driven gear 30 which is connected to a shaft 34 and to a driven disk 36. The driven disk 36 engages with the drive disk 8 as will be explained in more detail below. The driven gear 30 meshes with a free gear 32 which is rotated by the driven gear 30. A shaft O-ring 38 seals the shaft 34 adjacent to the driven disk.

In the embodiment, the gears 30,32 are helical gears rather than conventional cross-cut gears to reduce noise and to reduce power needed to drive them.

The pump unit 20 has a cover 40 which extends over the cavity block and hence seals the upper face of pump cavity 24. A spring detent 42 is provided which engages with flange 16 in the drive unit to clip the pump unit 20 onto the drive unit 2. The pump unit 20 can be removed by compressing the spring detent 42. This enables the pump unit 20 to be replaced.

Fluid passes between inlet 26 and pump cavity 24 and between the pump cavity 24 and outlet 28 along channels 44 in cavity block body 22. Fluid is introduced into the channels 44 not through the pump body 22 but through the cover 40. Fluid passes from the channels 44 in the cavity block 70 into the pump cavity 24 through shallow channels 52 on the underside of cavity block 70. (FIG. 2).

The cover 40 is fixed to the body 22 using screws 54.

The cavity block 70 is located in a corresponding cavity hole 72 in pump body 22. The pump cavity 24 is provided in cavity block 72. This saves manufacturing costs since only the cavity block 72 needs high precision machining, and the same cavity block 72 can be used in embodiments with different numbers of pump units as illustrated in FIGS. 1 and 3.

The cavity block 70 is sized to have a lateral clearance 71 (FIG. 4), for example 0.1 mm to 1 mm, within the cavity hole 72 in pump body 22 so the cavity block is free to move laterally (perpendicular to the axis). Bearing 80 is provided on shaft 34 adjacent to O-ring 38 and this bearing, which is mounted in pump body 22 in a recess which locates shaft 34 and hence gear 30 and thus the cavity block 70 itself. This structure means that accurate fitting of the cavity block 70 in pump body 22 is not required.

The cavity block 70 is manufactured of material that is specially chosen. The base material, PTFE, has a number of suitable properties, namely chemical resistance, heat resistance and low friction. PTFE is able to creep, which has both advantages and disadvantages. The advantage is that the PTFE can creep to accurately fit round a shaft and hence provide a good seal. The disadvantage is that the creep can cause the accurate shape to deform.

The inventors have found that by filling PTFE with a filler, including glass or bronze, the bulk of the body of the material is prevented from creeping but there still remains the possibility of creep at the surface to enable the surface to deform and seal.

Thus, the material preferred is filled chemically resistant plastic. In the embodiment, bronze-loaded PTFE is used which the inventors have found to have excellent wear properties.

The cavity block 70 is held and sealed by an upper shim 74 with holes 76 corresponding to channels 44 and lower shim 78. These shims 74, 78 are in this embodiment made of PTFE and not softer elastomer to accurately define the upper and lower ends of cavity 24 since the height of the cavity is important for good pumping action—if the height of cavity 24 is too small the shims will rub against the gears 30,32 causing excessive wear and excessive power consumption but if there is too large a gap between the gears 30,32 and the shims 74, 78 the gear pump will be unable to achieve suitable pressures.

The inlet 26 and outlet 28 are in the form of tube fixings for fixing to flexible tubes that extend through cover 40 and the holes 76 in upper shim 74 (FIG. 4) into the channels 44. Flexible (plastic) tubes can be mounted to the inlet and outlet for use.

The pump is designed to be a micropump, i.e. the housing is no bigger than 20 cm in the maximum direction and preferably no bigger than 10 cm. The pump is designed to be portable and low power. It is also important that the replaceable pump unit 20 can be manufactured at low cost. Since the pump may be used in medical applications, reliability is also important.

These desiderata are difficult to achieve together, and a number of design features are used to address some of these aspects.

Firstly, it is useful that the gears 30,32 are retained within the pump cavity without the need to provide a spindle. This is achieved using a pump cavity 24 in the form of a figure of 8 as illustrated in FIG. 2.

However, the inventors have found that for the cavity 24 to correctly retain the gears the central point 50 of the figure of 8 shape is particularly important.

However, fluid needs to be introduced here. Therefore, the fluid is brought from the end of the channel 44 adjacent to the cavity 24 to the cavity 24 along shallow channels 52. These are no thicker than 30% of the thickness of the cavity block 70 and so retain central point 50 (FIG. 2) to retain correctly the gears 30, 32. Preferably, these shallow channels are less than 20%, further preferably less than 10% of the thickness of the cavity block 70.

A further design feature is the shape of the drive disk 8 and the matching shape of driven disk 36. It is important that these reliably mesh to provide a strong drive without risking that the pump unit 20 cannot be clipped onto the drive unit 2 if the disks 8, 36 are misaligned.

The drive disk has a pair of opposed teeth 56, each of which has a vertical end face and an inclined top face. The driven disk has the same shape.

Accordingly, if the pump unit is placed onto the drive unit with the teeth 56 of the pump unit interfering with the teeth 56 of the drive unit, the inclined top faces engage to rotate the disks to allow the placement of the pump unit. When the motor is switched on, the end faces of the teeth engage to provide a reliable and strong drive.

The O-rings 38 seals the shaft and the pump unit 20 on drive unit 2. To provide a suitable resilience in a small O-ring, a soft O-ring is used, typically with a Shore hardness of 30 to 60 instead of 70 to 90 for a conventional O-ring. The embodiment uses a material with a Shore hardness of 40 to 50.

The motor 6 is free to move laterally in the housing 4. In other words, the rotor is in a sense floating. The design of the drive disk 8 and driven disk 36 is such that when the motor is operated, the motor will tend to centre. In other words, there is a free play of about 0.1 mm to 0.3 mm, in the embodiment 0.2 mm, which allows the motor 6 to laterally move to align the axis of the motor with the axis of the driven gear 30. By aligning the axes accurately in this way, the noise is reduced as is the power needed to drive the motor thereby reducing noise and increasing battery life.

The upper shim 74 is a polytetrafluorethylene (PTFE) shim. This has two functions. It is very low friction allowing the gears to rotate with minimum friction loss. It also functions as a seal. In the embodiment shown, the shim 76 is provided in matching recess in cover 40. Lower shim 78 fits in the cavity hole 72 in pump body 22.

If the height of cavity 24 is too small the shims will rub against the gears 30,32 causing excessive wear and excessive power consumption but if there is too large a gap between the gears 30,32 and the shims 74, 78 the gear pump will be unable to achieve suitable pressures.

By using this construction, the shape of pump body 22 is greatly simplified so this component can be manufactured with lower precision than cavity block 70.

FIG. 1 also illustrates a separate housing bottom 102 attached to the underside of housing 4 and allowing the motor 6 to be introduced from the underside, as well as circuit board 104 with connectors 106 for electrical connection to motor 6.

In this way, a reliable pump system is provided with a reusable pump unit 20 that can be manufactured relatively cheaply. The unit may be small enough, typically of order 10 cm, that it is easily portable and may even be battery driven.

The use of separate top pieces 12 and cover 40 to cover the drive unit 2 and body 22 allows for easy manufacture.

The skilled person will realise that all or some of these details may be changed. For example, instead of the use of screws 54 glue may be used to fix the cover 40 on the pump unit body 22. Alternatively, a press fit or snap fit may be used.

Other types of pump may also be used, if appropriate. The gear pump described above is particularly suitable if the pump needs to operate with significant back pressure but if this requirement is relaxed other designs may be possible.

The number of teeth 56 may be varied and more than two teeth on each disk may be used if desired.

Note that the channels 44 and cavity 24 in cavity block 70 extend through the cavity block 70 with constant cross section. This greatly eases manufacture to high tolerance and makes it easy to check the positions of the channels 44 and cavity 24 to ensure reliable pump operation.

The number of motors 6 and pumps may be varied. FIG. 1 shows a single motor and a single pump but FIG. 2 shows a variation with two motors and pump units. Note that the same cavity block 70 may be used for both arrangements. This allows only a single cavity block 70 to be manufactured for different numbers of pumps. Since the cavity block 70 is the part that needs to be manufactured most accurately, this saves in manufacturing costs.

In the embodiments shown, all parts are manufactured in plastics by injection moulding but it may also be possible to manufacture individual components in different ways. In particular, the cavity block 70 may be machined if required.

In an alternative embodiment, the top piece 12 is omitted and the top of the motor is part of the housing. In this case, the bottom plate of the housing may be omitted to allow introduction of the motor 6 into the housing 4 during manufacture. The motor may then be sealed with a flexible material such as glue or a further O-ring at its base. In this case, even if the bottom of the motor is held in place the top of the motor may still have enough flexibility of movement to allow automatic centring if the seal has sufficient flexibility to allow small angular deviations.

A further alternative embodiment uses a cavity block 70 without requiring a removable drive unit as illustrated in FIG. 4. In this embodiment, a cheap to manufacture gear pump is provided for applications where it is not necessary to replace part of the pump for reasons of hygiene or otherwise. As in the embodiments above, the gear pump is defined by pump cavity 24 in a cavity block 70 mounted in a cavity hole 72 in pump body 22. Cover 40 is screwed on to the pump body with screws 54 and inlet 26 and outlet 28 provided on cover 40.

FIG. 4 illustrates a further feature. In this embodiment, lower shim 78 is replaced with pad 80. A number of alternative cavity blocks 70 and pads 81 may be provided, each with the same total height. In this way, a number of different pumps may be manufactured simply by replacing the gears 30,32, cavity block 70 and pad 81 without requiring a large variety of different parts.

The motor 6 is a brushless motor with shaft 82 passing through. Motor coils are integrated into motor housing 84 which has a motor cavity 86 for accepting brushless drive unit 88 attached to shaft 82. The motor housing is fixed onto pump body 22 with screws 90.

Between the pump body 22 and hence the pump cavity 24 in cavity block 70 and the motor cavity 86 are provided O-ring 92, rotary seal 94, bearing 96 and spacer 97 arranged in that order on the shaft 82. A second bearing 98 and second spacer 97 is provided to locate the opposite end of the shaft 82, the second bearing 98 locating in the motor cavity 86 so that the shaft 82 is supported at each end. The first bearing 96 locates in the pump body 22.

In embodiments, the rotary seal 94 may be omitted. In this case, suitable for use in pumping non-corrosive fluids such as water, the pumped fluid may flow into and fill the motor cavity 86 to provide lubrication. Such embodiments may have extended life since rotary seals may be the first component to wear out when a pump of this type is used.

Of course, where there is a requirement to contain the pumped fluid, the rotary seal 94 may prevent egress of the pumped fluid.

Those skilled in the art will realise that there may be many variations of the embodiments described above and that the materials and spatial arrangements may be changed. 

1. A pump system comprising: a pump body having a pair of gears located in a pump cavity and defining channels; an inlet and an outlet connected to the channels of the pump cavity; and a motor connected to a longitudinal shaft for driving the gears; wherein one of the gears is a free gear located in the pump cavity by the walls of the cavity, and the pump body defines a cavity hole and the pump cavity and channels are defined in a cavity block mounted in the cavity hole, the cavity block being free to move laterally in the cavity hole.
 2. A pump system according to claim 1, further comprising channels through the cavity block connecting the inlet and the outlet to the pump cavity, further comprising a shallow channel of thickness no greater than 30% of the thickness of the cavity block between the channels and the pump cavity to introduce fluid into the pump cavity and to extract fluid from the pump cavity.
 3. A pump system according to claim 1 wherein the pump cavity and channels extend through the cavity block with constant cross section.
 4. A pump system according claim 1, further comprising a pad having the same cross section as the cavity block, the pad being mounted adjacent the cavity block within the cavity hole.
 5. A pump system according claim 1 comprising: a drive unit enclosing the motor; a pump unit including the pump body, the cavity block in the pump body, the inlet, the outlet and the gears; wherein the pump unit is a removable pump unit which may be separated from the drive unit.
 6. A pump system according to claim 1, wherein the pump unit further comprises a driven disk, the driven disk being connected to the longitudinal shaft and facing the drive unit, the longitudinal shaft passing through the pump body, further comprising a drive disk arranged on motor in the drive unit and meshed with the driven disk.
 7. A pump system according to claim 1 further comprising a detent in the pump unit arranged to engage with a flange on the drive unit to hold the pump unit on the drive unit.
 8. A pump system according to claim 1, wherein the motor comprises a motor housing defining a motor cavity therein cooperating with a drive unit on a shaft, the drive unit being located in the motor cavity.
 9. A pump system according to claim 1 further comprising a PTFE shim between the gears in the pump cavity and the cover, the PTFE shim sealing the top of the pump cavity.
 10. A pump system according to claim 1, wherein the pair of gears are helical gears.
 11. A pump system according to claim 1, wherein the motor is free to move laterally within a motor housing by at least 0.1 mm.
 12. A pump system according to claim 1 wherein the cavity block is of plastics containing filler.
 13. A pump system according to claim 12 wherein the cavity block is of bronze-loaded PTFE. 